1. Field of the Invention
The present invention relates to a method and a device for generating a reference signal in a cellular mobile communication system having a plurality of base stations. More particularly, the present invention relates to a method of efficiently generating a reference signal in a Distributed Antenna System (DAS) in which antennas operated by each base station are distributed at a service area of a corresponding base station, and a device thereof.
2. Description of the Related Art
A mobile communication system according to the related art provides a voice-oriented service. However, a current mobile communication system has been developed to high speed and high quality wireless packet data communication system to provide a voice service, a data service, and a multi-media service. Various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), LTE-Advanced (LTE-A) of the 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) of the 3GPP2, and 802.16 of the Institute of Electrical and Electronics Engineers (IEEE) have been developed in order to support a high speed and high quality wireless packet data transmission service. More particularly, an LTE system is a system developed to efficiently support high speed wireless packet data transmission. The LTE system may maximize capacity of a wireless system using various warless access technologies. The LTE-A system is an evolved wireless system of an LTE system and has improved data transmission performance compared to the LTE system.
A 3G wireless packet data communication system, such as HSDPA, HSUPA, and HRPD uses an Adaptive Modulation and Coding (AMC) scheme and a channel reply scheduling scheme to improve transmission efficiency. When the AMC scheme is used, a transmitter may control transmitted data according to a channel state. For example, if the channel state is not excellent, the transmitter may reduce the amount of the transmitted data to reduce a reception error probability to a desired level. If the channel state is excellent, the transmitter may increase the amount of the transmitted data to efficiently transmit a large amount of information while maintaining a reception error probability to a desired level. When a channel reply scheduling resource management method is applied, a transmitter selectively provides a service to a user having an excellent channel state among a plurality of users. Accordingly, available system capacity in the channel reply scheduling scheme is increased compared to a scheme of allocating a channel to serve one user. Such capacity increase refers to a multi-user diversity gain. According to the AMC scheme and the channel reply scheduling scheme, a transmitter may receive feedback of partial channel state information and apply a suitable modulation and coding scheme at the most efficiently determined time point.
When the AMC scheme is used together with a Multiple Input Multiple Output (MIMO) transmission scheme, it may include a function of determining the number of spatial layers of a transmitted signal or a rank. In this case, the wireless packet data communication system to which the AMC scheme is applied simply considers a code rate and a modulation scheme to determine an optimal data transmission rate and a number of transmission layers using the MIMO.
In recent years, research has been actively performed to convert the Code Division Multiple Access (CDMA) scheme, which is a multiple access scheme used in 2G and 3G mobile communication systems, to an Orthogonal Frequency Division Multiple Access (OFDMA) scheme, which is a next generation system. 3GPP and 3GPP2 have begun to standardize an evolved system using OFDMA. It is known in the art that capacity increase is expected in an OFDMA scheme compared to a CDMA scheme. One of various reasons causing capacity increased in the OFDMA scheme is that a frequency domain scheduling may be performed in a frequency axis. As a capacity gain is acquired through the channel replay scheduling scheme due to channel change characteristic according to time and when a channel is changed according to a frequency, a higher capacity gain can be obtained.
In a case of the related art, a cellular mobile communication system having a plurality of cells is established as illustrated in FIG. 1 to provide a mobile communication service using the various schemes.
FIG. 1 illustrates a cell structure having three cells where transceiving antennas are arranged in centers of the cells, respectively, in a cellular mobile communication system according to the related art.
Referring to FIG. 1, the cellular mobile communication system includes three cells, that is, a cell 100, a cell 110, and a cell 120, and reference numeral 160 illustrates the cell structure of the cell 100. The cell 100 includes a centrally located antenna(s) 130, a User Equipment 1 (UE1) 140, and a UE2 150 as illustrated in the cell structure 160. The antenna 130 provides a mobile communication service to two UEs located in the cell 100. Since the UE1 140 receiving the mobile communication service using the antenna 130 is located away from the antenna 130 compared to the UE2 150, transmission speed of supportable data is relatively low.
As shown in FIG. 1, antennas of respective cells 100 have a form of a Central Antenna System (CAS) arranged at a center of a corresponding cell. In a case of the CAS, although a plurality of antennas are arranged in every cell, the antennas are arranged at the center of the cell so that communication with respect to a service area of the cell is performed. As shown in FIG. 1, when antennas by cells are arranged and operated in the form of the CAS in the cellular mobile communication system, in order to measure a downlink channel state for each cell or demodulate a downlink signal, a reference signal needs to be transmitted. In a case of a 3GPP LTE-A system, a UE estimates channel information used to demodulate the downlink signal, and measures a channel state between the UE and a base station using a Channel Status Information Reference Signal (CSI-RS) transmitted by the base station.
FIG. 2 illustrates a location of a CSI-RS which a base station transmits to a UE in an LTE-A system according to the related art.
Referring to FIG. 2, a signal with respect to four Demodulation-Reference Signal (DM-RS) ports in each location corresponding to reference numerals 220 and 221 may be transmitted. For example, signals with respect to DM-RS ports 7, 8, 11, 13 are transmitted to a location corresponding to the reference numeral 220, and signals with respect to DM-RS ports 9, 10, 12, 14 are transmitted to a location corresponding to the reference numeral 221. Different DM-RS ports corresponding to the same location are identified through a Code Division Multiplexing (CDM) scheme. Codes allocated to the DM-RS ports are defined as illustrated in Table 1.
TABLE 1Antenna port p[ wp(0) wp(1) wp(2) wp(3)]7[+1 +1 +1 +1]8[+1 −1 +1 −1]9[+1 +1 +1 +1]10[+1 −1 +1 −1]11[+1 +1 −1 −1]12[−1 −1 +1 +1]13[+1 −1 −1 +1]14[−1 +1 +1 −1]
FIG. 3 illustrates a mapping scheme of a sequence index with respect to (x) DM-RS resources per Resource Block (RB) by DM-RS ports as a frequency preference mapping scheme according to the related art.
Referring to FIG. 3, a mapping scheme of a sequence index with respect to 12 DM-RS resources per RB by DM-RS ports is illustrated as a frequency preference mapping scheme.
A DM-RS sequence transmitted from locations of DM-RS ports is defined by Equation 1.
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                                                          ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              N            RB                                          m                ⁢                                                                  ⁢                ax                            ,              DL                                -          1                                    Equation        ⁢                                  ⁢        1            
In Equation 1, NRBmax,DL indicates the number of RBs available in a downlink signal and m indicates an index of a sequence. The c(i) is a pseudo-random sequence, and an initial value with respect to a generator of the pseudo-random sequence is defined as illustrated in Equation 2.cinit=(└ns/2┘1)·(2NIDcell+1)·216+nSCID  Equation 2
In Equation 2, NIDcell represents a cell ID, and n Service Channel IDentifier (nSCID) represents scrambling identity information, and is determined as 0 or 1 by a scrambling identity field in a Data Character Identifier (DCI) format 2B or 2C transferred through a Physical Downlink Control Channel (PDCCH). For example, antenna ports in the same cell have the same cell ID, and two types of DM-RS sequences for each antenna port are identified by nSCID.
Referring again to FIG. 2, a signal with respect to two CSI-RS antenna ports in each location of reference numerals 200 to 219 may be transmitted. For example, the base station transmits two CSI-RSs for measuring downlink to the UE from a location 200. As shown in FIG. 1, in a case of a cellular mobile communication system having a plurality of cells, locations to which CSI-RS is transmitted may be allocated differently according to cells. For example, in a case of a cell 100 shown in FIG. 1, a CSI-RS may be transmitted from the location 200 in a case of the cell 100 shown in FIG. 1, from the location 205 in a case of the cell 110, and from the location 210 in a case of the cell 120. As described above, a resource for CSI-RS transmission is allocated in different locations with respect to respective cells to prevent CSI-RSs of different cells to cause mutual interference.
A CSI-RS sequence transmitted from locations of CSI-RS antenna ports is defined by Equation 3.
                                                        r                              l                ,                                  n                  s                                                      ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                                                          ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              N            RB                                          m                ⁢                                                                  ⁢                ax                            ,              DL                                -          1                                    Equation        ⁢                                  ⁢        3            
In Equation 3, c(i) is a pseudo-random sequence, and an initial value with respect to a generator of the pseudo-random sequence is defined as illustrated in Equation 4.cinit=210·(7·(ns+1)+l+1)·(2·NIDcell+1)+2·NIDcell+NCP  Equation 4
In Equation 4, 1 notes an Orthogonal Frequency Division Multiplexing (OFDM) symbol order in one slot, and NCP is determined with 0 or 1 according to a length of a Cyclic Prefix (CP) used in a cell.
When using a CAS scheme shown in FIG. 1, as transceiving antennas are centrally located at the cell, there is limitation in supporting a high transmission rate to a UE spaced apart from the center of the cell. For example, in the CAS, providing a high speed data transmission rate to UEs existing in a cell is greatly affected according to where the UE is in the cell. Accordingly, in the cellular mobile communication system, a UE located relatively close to a cell center may transmit and receive data with a high transmission rate. A UE located in a remote location may not transmit and receive data with high transmission rate.
Therefore, a need exists for a method and a device for generating a reference signal in a cellular mobile communication system having a plurality of base stations.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.